As an healer marks make it easier or "possible" to throw in damage or CC. As a DPS having marks make it quicker to focus or to not. As a tank it makes it way easier to control the enmity. Tank doesn't have to be the one to mark everything, just put a Shield Lob/Tomahawk macroed with marks and you're done. It's not like you can't play markless but they help a lot, especially with players you doesn't know so do it